This invention relates to antenna feed systems, and more particularly to systems for deriving individual signals from a number of signals received within a given band at an antenna system, which signals may be subject to adjacent channel interference.
The rapidly growing use of localized antenna systems for recieving signals from communications satellites has directed increasing attention to improvement of the cost and performance of these systems. Problems have arisen in obtaining the proper signal-to-noise ratio because adjacent or co-channel interference can become excessive, and can only be eliminated in the prior art by the use of specialized or expensive waveguide transmission or amplification equipment.
In satellite communication signals that are devoted to television transmissions, for example, an available frequency band (e.g. 3.7 to 4.2 GHz) is divided into 12 or 24 different channels, each channel of 36 MHz bandwidth occupied by different transmissions, such as one television and two audio carriers or subcarriers. The constraints on the transmitting system, the satellite, are fixed in that only a limited amount of power is available for concurrent transmission of all of these signals. The constraints on the receiving system then arise both from performance and cost considerations. To receive a high quality television picture, for example, it is required to have a signal-to-noise ratio of approximately 40 db on reception. External noise is combated by using frequency modulation with threshold extension, although other techniques are also available, and internal noise at the receiving end is minimized through the use of low noise amplifiers. The systems have become economically feasible for widespread use because of the development of GaAsFET devices, which cost far less than the parametric amplifiers and similar systems that would otherwise have to be used. Even using GaAsFETs, the signal-to-noise ratio is still marginal, so co-channel noise is combated by the use of crosspolarization of adjacent channels in 24 channel satellites since adjacent channels are spaced 20 MHz apart. Crosspolarization, however, introduces its own penalties in the form of a need to distinguish between polarizations, whether amplifying one selected channel by itself, or amplifying all of the channels concurrently. The techniques heretofore available for providing polarization selectivity have heretofore introduced unwanted expense or unreliability, or both, into the systems.
It was recognized early that cross-polarized signals in different channels received at a microwave antenna could be separately extracted by using an antenna feed comprising a feed waveguide, a polarization responsive waveguide junction, and output waveguide structures coupling from the waveguide junction to associated amplifiers. The components used in this system are not only expensive, but extremely bulky and cumbersome, so that when used as a prime focus feed relative to a parabolic antenna they are both much bigger and much heavier than desirable. In addition, their angle of rotation relative to incident polarized energy cannot be changed except by the use of a large and heavy mounting. Consequently, other systems are also currently in use. One approach is to use only a single polarization feed, so that only one of the two alternate sets of channels can be received. An obvious variation that is also used is to employ a single polarization feed and to physically rotate the feed through 90.degree. in order to switch between sets of channels. It was also recognized that an equivalent result could be obtained more conveniently by mounting the feed on a standard antenna rotor that could be remotely controlled. Such an arrangement is quite slow and because the large units must be exposed to the weather, they tend to experience corrosion and mechanical difficulties and therefore have proven to be quite unreliable.
Other physical and electrical requirements of these systems must also be borne in mind. The feed that is typically employed is a prime focus feed, mounted at the focal point of a parabolic reflector of 3-4 meters or more in diameter. The Cassegranian-type feed is also used. In either event, it is desirable for low noise amplifiers to be at the feed itself. Thus, transmission line losses in a relatively long line coupling to an adjacent facility where processing equipment is located are not apt to present a problem. Further, any of a number of different combinations of channel selection, conversion and receiver equipment, may be used and the antenna feed system should be readily adaptable. Systems heretofore known have not permitted this versatility in a price range that is suitable for the current users of these communications channels.
In addition to the art mentioned above, it is noted that complex amplification systems have been used for cancelling cross-polarization interference, as evidenced by U.S. Pat. No. 4,283,795. Horns having internal ridges are being used for transmission of cross-polarized signals at high power levels to different ones of the receiving systems mentioned above. Such arrangements do not, singly or in combination, suggest the manner in which a superior antenna feed system overcoming the problems of the prior art may be provided.